Magnetic resonance imaging, nuclear magnetic resonance imaging, or magnetic resonance tomography are medical imaging techniques used in radiology to visualize detailed internal structures—the term “MRI” will be used herein to reference all of these and related technologies. MRI makes use of the property of nuclear magnetic resonance to image nuclei of atoms inside the body.
An MRI machine uses a powerful magnetic field to align the magnetization of some atoms in the body, and radio frequency fields to systematically alter the alignment of this magnetization. This causes the nuclei to produce a rotating magnetic field detectable by the scanner, and this information is recorded to construct an image of the scanned area of the body. Strong magnetic field gradients cause nuclei at different locations to rotate at different speeds. 3-D spatial information can be obtained by providing gradients in each direction.
MRI provides good contrast between the different soft tissues of the body, which makes it especially useful in imaging the brain, muscles, the heart, and cancers compared with other medical imaging techniques such as computed tomography (CT) or X-rays. Unlike CT scans or traditional X-rays, MRI uses no ionizing radiation. The very high strength of the magnetic field however, can cause “missile-effect” accidents, where ferromagnetic objects are attracted to the center of the magnet, and there have been incidences of injury and death due to the strong magnetic fields generated by the MRI device. To reduce the risks of projectile accidents, ferromagnetic objects and devices are typically prohibited in proximity to the MRI machine and patients undergoing MRI examinations are required to remove all ferrometallic and ferromagnetic objects from their vicinity and apparel. Ferromagnetic detection devices are used by some sites in an attempt to avoid the introduction of ferromagnetic objects into the vicinity of the MRI machine, as well.
Moreover, in addition to possibly adverse physical movement of ferromagnetic objects located in proximity to the MRI machine, such objects may also provide image artifacts and/or geometric distortions which can cause difficult-to-read or even misleading MRI test results. During imaging it is also possible for metallic objects inside the MRI machine to heat up due to eddy currents, and this concern for patient safety also constrains material selection for MRI-guided devices. Due to the desire of the user to have highly accurate and precise MRI images, such unwanted side effects of ferromagnetic, ferrometallic, and/or metallic presence may prevent certain procedures from being guided by MRI. Additionally or alternatively, MRI images obtained in the presence of ferromagnetic objects may be unsuitable and need to be repeated later (if possible), which can cause unwanted expense and/or delay in treatment of a patient.
Thermal ablation is an interventional technique that helps to enable percutaneous treatment of many cancers and other disorders throughout the human body. Acoustic ablation can help steer thermal energy electronically and is known to be MRI-compatible. Therefore, acoustic ablation may be amenable to real-time thermal dose monitoring via MRI-assisted thermometry.
In the neurological field, active cannulae are sometimes used for percutaneous interventions, such as for acoustic and other thermal ablations, biopsies, deep brain stimulation, electrode placement, or for any other desired reason(s). An example of an active cannula is given in U.S. Patent Application Publication No. 2009/0171271, published Jul. 2, 2009 by Robert James Webster et al. This and other active cannulae include a plurality of concentric or nested tubes which may each have preformed curvatures and/or predefined flexibilities. The translation and/or angular orientation (rotation) of each tube may be controlled individually such that the tubes can telescope and twist to move the tip of the cannula into a desired orientation along a desired path. The tip of the cannula may contain, carry, orient, or otherwise provide positional assistance to an “end effector”, which is a tool such as a biopsy gun, ablator, electrode, electrode positioner, camera, fiber-optic lens, or any other suitable tool which may be used to perform some task at, and/or have some effect upon, an area of the patient tissue to which the active cannula carries this “end effector”, preferably in a precise and accurate manner for most use environments of the present invention.
It may be desirable for an active cannula to be precisely steered, moved, and controlled in certain use environments, particularly in neurosurgical interventions, such as, for example, in the application of ablative energy. The desired precise motion of the active cannula may be facilitated robotically in surgical environments, as robots can be controlled to sub-millimetric precision. MRI imaging and guidance would be helpful in observing and directing the movement of the active cannula, but the robots currently available for cannula guidance are lacking in MRI-compatibility. Currently available robots actuated by piezoelectric motors cannot truly operate in real time in an MRI environment because the high voltage power supply to the motors cannot be energized while the MRI machine is imaging due to potential distortion effects. Furthermore, MRI-compatible robots employing piezoelectric or hydraulic actuation are not easily integrated into existing hospital facilities due to the unfamiliarity of maintenance and operating personnel with these power sources. In addition, hydraulically-actuated MRI-compatible robots require a steady supply of hydraulic fluid, which both is not generally available in a hospital environment (thus, must be separately stocked and managed) and also can cause a mess, slipping hazard, or even contamination of a patient's body if it leaks out of the robot system within the hospital environment.